Temperature control system



April 25, 1939'. P. F. SHIVERS TEMPERATURE CONTROL SYSTEM 2 Sheets-Sheet 1 Filed Oct. 31, 1935 muamooum J r GK.

PauZ Va'verw L mobs-D2- I. IVAN hmu L April 25, 1939. P SHWERS 2,155,984

TEMPERATURE CONTROL SYSTEM Filed Oct. 51, 1955 2 Sheets-Sheet 2 O kl N SEN I Paul kiven;

TO TRANSFORMER \2\ Patented Apr. 25, 1939 UNITED STATES PATENT QFFIcE TEMPERATURE CONTROL SYSTEM Application October 31, 1935, Serial No. 47,72?

have the disadvantage that they are dependent upon the voltage of a power supply being maintained comparatively constant. Where there was any serious fluctuation in the voltage of said power supply, these devices have failed to indicate and control the temperature properly.

An object of the present invention is to provide a temperature controlling means employing the use of a photo-electric celiwhich temperature controlling means is independent of voltage variations in the power supply.

A further object of this invention is to provide 25 a temperature controlling system employing the use of two cooperating self-balancing systems, one of which is controlled by the temperature and the other of which controls temperature varying means.

Further objects of the invention will be apparent from the accompanying specification and drawings, of which Figure l is a-schematic view of my preferred species and of which Figure 2 is a schematic showing of a modification thereof.

A furnace or other heating means is designated by the reference numeral i0. Located in said furnace is an object II which is heated thereby. The furnace is supplied with fuel through a pipe l2 which is controlled by a valve l3.

A portion of my control system which is directly controlled by the temperature of object II is shown in the left-hand portion of the. figure. This-portion of my system comprises two photoelectric cells It and it, of which cell I4 is located to receive light from the hot body and cell I! to receive light from an electric lamp IS. The two cells constitute two arms of a Wheatstone bridge across which is connected at thermionic amplifier H, the grid circuit of which forms a bridging connection across the two halves of the electrical bridge. The system is designed so as to be balanced when the amounts of light supplied to the cells bear a predetermined ratio to each other. Any decrease in the ratio of the light supplied to cell It to that to cell It by reason of the temperature of the hot body rising, causes the potential of the grid to be increased, while an opposite change in the ratio causes a decrease in the potential of the grid. The output of amplifier tube i1 is applied to a second thermionic amplifier IS, the output of which is to beemployed to energize lamp I6. If the temperature of the hot body is increased, the ratio of the amounts of light supplied to cells [4 and i6 respectively, will be decreased, thus causing an increase in the output current of amplifier 11, which is amplified through amplifier l8 and causes an increase in current flow through lamp I6 which is sufilcient to restore the previous ratio in the amounts of lights supplied to the two cells. This increase in flow of current to the lamp I6 serves as a means for indicating the temperature and for operating a portion of my control system which operates the temperature varying means. This portion of my system is shown in the right hand section of the drawings The last mentioned portion of my control system broadly comprises a balanced relay switch 69 which is unbalanced upon a change in the flow of current to the lamp [6. Said switch l9 upon being moved to either of two closed positions by such an unbalanced condition causes a reversible motor 20 to rotate in either of two directions. This motor controls the adjustment of valve 13 and controls a rheostat 22 which acts to rebalance switch It moving it again to an open position.

Referring more particularly now to the details of my apparatus, I obtain direct current for the operation of the photo-electric cells and the various amplifier tubes through rectifier-filter systems 25 and 26. These rectifier-filter systems are supplied with alternating current by a transformer 2'! having a plurality of primaries 28 and 29 connected to line wires 30 and 3| which are connected to a suitable commercial source of alternating current. Said transformer 21 also comprises a plurality of secondaries 8 and 9 which are connected to the respective rectifier-filter systems 25 and 28. The rectifier-filter systems 25 and 26 are of the type well known in the art and each comprises a full wave thermionic rectifier tube 32, a plurality of condensers 33 connected across the output of said tube and a choke coil 34 connected in the output circuit between the points at which the condenser is connected. Condensers 33 and choke coils 34 serve to filter the output of the rectifiers and to almost entirely eliminate the pulsations in the current supplied by said amplifiers, in a manner well known in the art.

The way in which my power supply unit diirers from the conventional type is in the employment of a separate transformer for supplying heating current to the filament heaters 35 and 88. A transformer 81 is provided for this purpose. Bald transformer comprises a primary 38 and a plurality of low voltage secondaries 38, 4t and M. The primary 38 is connected to line wires 39 and at through a, voltage regulator 62. This voltage regulator comprises a resistance element es located in an evacuated receptacle so. The resistanoe element til is of a material whose resistance varies almost directly with the voltage applied to its terminals and so serves as a means for maintaining a constant current flow through primary 238 regardless of variations in the line voltage. The heating current for the filaments of all the thermionic tubes employed in my systern is controlled by this voltage regulator and in this manner ll saifesuard against the nossl bility of the operation of my system being adected by a variation in the temperature of the filaments caused by a voltage variation secondaries 89 and so of transformer ill supply heating current to the heating filaments and 8t respectively.

The output of the rectineroillter system is connected across a potentiometer which serves to supply current for the DEE-Ellutidil oi the photo electric cells M and it and for the oyeration of the amplifier it. The output oi? the rectifier-Elk ter system is connected across a potentiometer G6 which serves to supply current for the opera tion of the amplifier til. I

The photo-electric cells Ml and iii are con= nected in series with a portion of the potentiom= eter (it between tap 3?] and the negative terminal or through conductors 69, bit and ti, and ac== cordingly have a voltage corresponding to the potential drop between tap All and terminal 88 impressed across their terminals.

The amplifier ill is a conventional three-=elec== trodo thermionic vacuum tube. This tube is shown in the drawings as a tube oi the type in which the cathode is indirectly heated. The tube comprises the usual cathode 52, the grid '63 and a plate 5d. As previously indicated, the

cathode 532i is indirectly heated by means oi a heater This heater is supplied with current through a transformer comprising a line volt= age primary El and a secondary to. The pri= mary li'l is connected through conductors Ed in parallel with the primary lid in such a manner that all the current flowing to said primary Ell passes through the voltage regulator 62.

The grid 68 is connected through'conductor to with conductor 5i at a point intermediate oi the two photo-electric cells. The cathode 52 is con nected through a conductor ill with potentiometer at at a tap 62. It will be seen that the portion of the grid circuit comprising conductor Eli, cathode o2, grid 53 and conductor til constitutes a bridging connection across a Wheatstone bridge, two arms or which are constituted by the two photo-electric cells it and it and the other two arms of which are constituted by the two sections of potentiometer 55 between tap ll and tan t2 and between tap it? and terminal (it. So long as the bridge is balanced no voltage will be impressed upon the grid circuit. Upon the conductivity of photo-electric cell it increasing, however, the bridge will be unbalanced with the result that grid 53 is maintained at a potential lower than cathode 52. It will thus be seen that any variation in the relative conductivity of cells it and II will chance the potential on the arid circuit and thus change the flow oi. current in the plate circuit of tube l'l. Since it is desirable that the grid be always maintained at a lower potential than the cathode, the circuit is designed so that when the system as a whole is balanced for a temperature oi. the hot body Ii close to the desired value, the light received by cell it will be sumciently greater than that received by cell it, that the grid will be biased negatively to a point where the tube 55 is operating in the midpoint or the straight line portion of its characteristic curve.

The plate 5d of tube it is connected through a conductor (it with one terminal of a resistor t. The other terminal or said resistor is connected to the positive terminal d5 of potentiometer 55 through conductor E56. The portion of the notchtiometer irom terminal to to tap 62 determines the voltage applied to the plate circuit of tube it. The resistor to constitutes the load resistance oi the tube and serves as a coupling resistance between said tube and the amplifier it to be presently described. Since under a balanced condltion oi the system, the bias oi grid 63 is never so great to prevent how of plate current, current will normally flow through resistor 6 Thus, there is always some potential drop across resister 86 while the apparatus is in accretion.

amplifier i8 is shown as comprising two conventional three-electrode thermionic vacuum tubes 87!. These tubes each comprise a filament (38,, a grid 69 and a plate it. The filaments 68 are supplied with heating current from the secohdary ii oi transformer ill through conductors ii. Connected to a mid-point of secondary ii is a conductor "it which is connected to potentiorneter til at a tap T13. This connection serves in effect to connect the cathodes 6t to the potentiometer at the tap it. The grids lit are connected in parallel to the positive terminal of resister to through a conductor it. The negative terminal lift of potentiometer 46 is connected to the resistor 6d at the terminal opposite to that to which grids til are connected. The grid circuits oi the tubes 6? thus comprise the filaments 58, conductor "ii, conductor 12, the portion of potentiometer 6 between tap it and negative terminal it, conductor l8, resistor tt, conductor it and grid 69. It will thus be seen that the grids be are biased negatively with respect to the cathode 88 by a voltage corresponding to the voltage drop across the portion or potentiometer so between tap it and negative terminal 15 minus the voltage drop across resistor 6d. Thus, any variation in the current flow through resistor tit due to a change in the plate current of tube ill will affect the bias of the grid 63.

The output circuit of tuoes d'l is constituted by plates it, conductor ii, an indicator it, a recorder it, a conductor 88, lamp it, conductor 88, resister 82, conductor 88, the portion oi the pollentlometer 86 between the positive terminal 84 and tap l3, conductors l2 and Till and the cathode 8B. Thus any change in the output current of tube 6'? will cause a change in the flow of current through lamp it, through the indicator (8 and recorder 78, and through resistance 82. The portion of potentiometer at between the terminal 84 and tap 13 determines the voltage applied to the plate circuits of tube 68.

The portion or my system just described constitutes the self-balancing temperature responsive portion of my system. I will now describe the portion of my system which actuates as a temill aromas perature control means in response to any change in the previously described temperature responsive means.

As previously stated, switch I8 controls the action of a reversible motor 28 which controls the position of the valve I8 and also a rheostat 22 which in turn controls switch I8. This switch comprises a pivotally mounted switch arm 88 adapted to engage with either of two contacts 88 and 81. This switch arm has a horizontal arm 88 which is subjected to the action of three electro-magnetic coils 88, 88 and 8|. Coil 8| is supplied with current through a resistance 82, being connected in parallel with a portion thereof through a sliding contact 82 and conductor 88.

The coils 88 and 88 are supplied with current from a source which is entirely independent of the previously described temperature responsive portion of my system. Coils 88 and 8I are located on one side of the switch arm and coil 88 on the other side, and under normal conditions coil 88 exerts an equal and opposite force upon the switch arm to that exerted by coils 88 and 8I acting conjointly. When said switch is so balanced the switch arm 85 is in engagement with neither contacts 88 nor 81.

A step-down transformer 84 serves to supply low voltage current for the relay coils 88 and 88 and for the operation of the reversible motor 28. This transformer comprises a line voltage primary 85 and a low voltage secondary 88. A primary 88 is connected through conductors 81 with line wires 88 and 8|.

A rectifier is employed to rectify the current supplied by secondary 88 to the coils 88 and 88. This rectifier may be of any suitable type but is shown for purposes of illustration as a conventional bridge rectifier having input terminals 88 and I88 and output terminals I8I and I82. The input terminals 88 and I88 are connected to the opposite terminals of the secondary 88. Coils 88 and 88 are connected in series through conductors I88 and I84 to the output terminals IM and I82 of said rectifier. Also connected to said output terminals in parallel with coils 88 and 88. is a rheostat 22. This rheostat comprises a resistance I85 and movable contacts I88 and I81. Contact I81 is connected to the terminal I82 through conductor I88 and the terminal of resistance I88 opposite said contact I81 is connected to the other output terminal I8I of rectifier 88 through a conductor I08. Contact I88 is mounted on an arm I28 and is connected through conductor I28 with the junction of coils 88 and 88. The rheostat thus serves as a voltage divider as will be more fully explained later. Connected between conductors I 88 and I88 in parallel with rheostat 22 and with coils 88 and 88 is a condenser II8. This condenser serves to smooth out the pulsations of the current supplied by the recti- The reversible motor 28 comprises a plurality of armatures I II and II2 and a plurality of field windings H8 and H4. One terminal of field winding H3 is connected to conductor II5 with contact 88 and one terminal of field winding II 4 is connected to a conductor II8 with contact 81. A common terminal I I1 of the two field windings is connected through a conductor II8 with one terminal of the secondary 88 of transformer 84. The opposite terminal of the secondary 88 is connected to the switch arm 85 through a conductor II8 embodying suitable means I28 permitting ready extension and contraction of said conductor. It will be readily seen that when switch arm 88 is in engage'mentwith contact 88, field winding 1 I8 is energized and that when switch arm 85 is in engagement with contact 81, field winding II4 will be energized. The motor 28 will rotate in one or the other direction depending upon which field winding is energized.

The motor 28 is connected through suitable reduction gearing I2I with a shaft I22. This shaft is connected to the arm I28 associated with contact I88 and upon movement of said shaft causes contact I88 to move along the resistance I88. Mounted on said shaft I22 is a pinion I24 which is operatlvely associated with rack I28 and connected to the stem of valve I8. It will thus be seen that when motor 28 is rotated a simultaneous adjustment is made of valve I8 and of rheostat 22. In the arrangement of the apparatus shown in the drawings the motor rotates insuch a direction as to move the valve towards a closed position and to move contact I88 in the direction of contact I81 when field winding H4 is energized. The opposite action takes place when field winding H8 is energized.

As previously explained, contact I88 is connected through the arm I28 and through the conductor I 28 with the Junction of coils 88 and 88. Potentiometer 22 thus serves to divide the current flow between coils 88 and 88. Upward movement of contact I88 causes more current to be supplied to coil 88 and less to coil 88, whereas, a downward movement causes the opposite action. Thus the rheostat 22 serves as a means for rebalancing the switch I8.

Referring now to the operation of my system, let it be assumed that for some reason the temperature of body II rises above that desired to be maintained. This results in photo-electric cell I4 being subjected to more light than previously causing grid 53 to become less negative with respect to cathode 52. This causes an increase of current fiow in the output circuit of tube I1 with the result that a greater voltage drop occurs across resistor 84. As previously explained, the bias of the grids 88 of tubes 81 depends upon the difference in voltage drops occurring across the portion of potentiometer between tap 18 and negative terminal 15 and across resistor 84. Thus, an increase in the voltage drop across resistor 84 results in a decrease in the negative bias of grids 88 so that the plate current of tubes 81 is increased. This causes an increase in the current supplied to lamp I8, which in turn tends to decrease the potential of the grid and decrease the current to lamp I8. Since the effect of a small change in light received by the photo-electric cells produces a large change in the voltage drops across the same, the value of the light will be the controlling factor in determining the new balance. The result will be that under the new balanced condition which will be reached, the ratio between the amounts of light received by the two cells will difler very slightly from the original ratio although each cell will be receiving more light. This very slight and practically negligible change in the ratio of the amounts of light received by the two cells results in a substantial change in the ratio of the voltage drops across the same, which substantial change is the factor which results in a greater output of tubes I1 and 81, to permit a rebalanced condition in the sys= tem as a whole. Under this new balanced condition, the current flow through lamp I8 will be larger than previously, thus resulting in an increased current flow through the indicator 18,

the recorder 18 and resistance 82. The increase in current flow through indicator l8 and recorder 19 serves as a means for indicating the increase" in temperature of body it. The increase in current flow through resistance 52 serves as a means for operating temperature controlling means to restore the temperature of body ii to that desired.

The increased flow of current through resistance 82 results in an increased flow of current through coil iii. The result of this will be that the switch it is unbalanced and switch arm 85 will be moved into engagement with contact 86. When this occurs field winding H3 will be energized and the motor will rotate in. such a direction as to move rack B25 downwardly thus moving the valve toward a closed position and at the same time to move contact I08 in the direction of contacts lull. The latter action results in a decrease in the current iiowing through coil as and an increase in the current flowing through coil it. As will be obvious, this movement of contact Hi6 on resistance M will cause a condition to be reached in which the decreased current flow through coil HQ and the increased current flow through coil til compensate for the increase in the current flow through coil 99 due to the increase in the current flow through the lamp circuit it, thus rebelancing the switch. Thus, the effect of the increase in current flow in the plate circuit of tubes tll due to the increased temperature of hot body it is to cause a movement toward closed position of the valve it sup= plying fuel to the furnace ii and a rebalanoing of the portion oi the system controlling the valve.

It will be understood that this entire action of the system takes place ost instantaneously due to the rapid response of the various thermionic devices employed. Thus, upon any appreciable change in temperature of the body it, the various adjustments are almost instantaneously made resulting in an immediate correction of the undesirable condition and a rebalancing of the entire system.

As previously indicated, one of the marked advantages of my temperature control system over previous systems oi this type is the freedom from disadvantageous effects due to voltage variations in the power supply. If we consider, for ex ample, that the line voltage should drop it will be noted that the condition oi the bridge to which photo-electric cells it and It are connected will not be affected as a. direct result thereof since while there will be a voltage drop across potentiometer 56 this drop will affect each of the two portions involved in the bridge equally. The voltage drop will, however, reduce the plate voltages across both tube l? and the two tubes 8'? with the result that the current flow through lamp is will be reduced. This reduction in current flow through lamp it causes an unbalancing of the bridge which tends to increase the current flow through tube I7 and, consequently, the current flow through the output circuit of tubes Bl with the result that lamp it is energized more brightly. While the lamp current cannot be restored to exactly the same value it had before the change, since an increase in illumination of the lamp tends to cause the apparatus to decrease the lamp current, it will be restored to a value which diifers by an almost negligible amount from that existing before the line voltage dropped. This negligible diflerence in lamp current causes a sumcient voltage change in cell it to change the grid bias sufficiently to cause the tubes to deliver the substantially normal amount of current to the lamp in spite of the decrease in plate voltage. As previously indicated, the heating elements of the various tubes are supplied through a voltage regulator 42 sothat these remain unaffected by any variation in line voltage. Moreover, since coils 89 and 90 are located on opposite sides of arm 86 any variation in the line voltage will not appreciably aifect switch l9 since the current flow through both of these coils decreases proportionately. In order to avoid any possible unbalancing due to the fact that coils 89 and 90 are subjected to variation in line voltage and coil til is not, coils 89 and 90 are made with a relatively large number of turns as compared with cell 9i. ihus any variation in line voltage will result in a substantially equal action on each side of the switch am. It will thus be seen that my temperature control system is remarkably free from voltage variations even where they are relatively large.

In Figure 2 I have shown a modified form of my control system. In this figure, the furnace is designated by the numeral its, the hot body by the numeral .635, the fuel supply pipe by the numoral 832 and the control valve by numeral I33. Since the portion of my apparatus embodying the photo-electric cells and thermionic amplifiers, which constitutes the self-balancing temperature responsive system is identical with that of Figure l, and since the operation and structure thereof is obvious in view of the description of the same in connection with Figure 1, detailed description thereof will not be given in connection with this figure.

The coupling resistance in the lamp circuit in this figure is designated by the reference numeral ltd. This resistance is employed to supply current to a coil 635 which constitutes one of two electro-inagnetlc relay coils 535 and lit of a switch 62 of Figure 1. Switch i317 comprises in addition to said coils 886 and H86, a switch arm ltd adapted to engage with either contact its or Mil. A coil ltd is supplied with step-down transformer Ml through a rectifier M2, which for purposes of illustration is shown as a conventional bridge rectifier.

Transformer Mil comprises a line voltage primary M3 and a low voltage secondary its. Pri mary M3 is connected through conductors M5 and I46 with line wires M7 and let A rheostat E69 controls the flow of current to the rectifier from transformer Ml. Said rheostat its comprises a resistance I and a contact arm. l5l. One terminal of resistance 458 is connected through conductor 652 with one terminal of the secondary iii, and the contact arm it? is connected through conductor 553 with one input terminal N8 of the rectifier M2. The other end of secondary M4 is connected through conductor 8'58 with the other input terminal 958 of the rectifier M2. The opposite terminals of relay coil 036 are connected through conductors H4 and M5 to the opposite terminals its and 656 of the bridge rectifier I42. A condenser H5 is connected across said terminals in parallel with coil M8 to smooth out the pulsations of the current supplied by said rectifier.

A motor it? is provided to actuate the contact arm iti of the rheostat M2 and to actuate the valve I33. Said motor is of the reversible type and comprises two rotors i553 and i553, and associated field windings ltd and Iti respectively.

The motor is connected through suitable reducdirect current from a' iii corresponding in function to switch tion gearing I82 with a shaft I83 which is designed to rotate contact arm I8I. Also mounted on said shaft I83 is a pinion I88 operatively associated with a rack I88. This rack is integrally connected to the valve stem so that rotation of shaft I83 causes the position of the valve to be changed. V

'The adJacent terminals of field windings I80 and IN are connected through conductor I88 to one terminal of secondary I. The other end of secondary I is connected to switch arm I38. Contacts I38 and I areconnected to the outermost terminals of the field windings I80 and I8I respectively. If the switch arm I38 engages with contact I38, field winding I80 will be energized and the motor will rotate in a direction to cause upward movement of contact arm I21 and downward movement of rack I l I, whereas if the switch arm moves in the opposite direction the other field winding I3I will be energized and the motor will rotate in the opposite direction.

In the event of a change in temperature of the.

hot body I3I with the resultant change in fiow of current to the lamp circuit, the energization of coil I38 will be affected in the same manner as was the energization of coil 10 in my preferred species. This will result in the balance between coils I35 and I38 being destroyed thus causing switch arm I38 to move into engagement with contact I38 or contact I40. If we assume that the temperature of the hot body is increased, the energization of coil I38 will be increased due to an increase in the flow of current through the lamp circuit andswitch arm I38 will be moved to the left into engagement with contact I38. This will result in field winding I80 being energized, causing the motor to rotate in such a direction as to move rack I83 downwardly thus moving the valve towards a closedv position and decreasing the amount of fuel supplied to furnace I30. At the same time contact arm I8I is moved upwardly on resistance I80 with the result that more current is supplied to the rectifier and consequently more current is supplied to coil I38. This action will continue untilcoils H8 and H8 are again balanced and switch arm H8 is again in its central open position.

While this species of my invention is not as free from voltage variations as my preferred species in that the current through coil I I8 is not subject to voltage variations whereas the current through coil H8 is, it nevertheless functions very satisfactorily where large voltage variations are not encountered, or where the error due to such a voltage variation in the temperature controlling system is not of serious consequence. In this case, as in the preceding one, the temperature control system which I have devised employs two self-balancing systems thus insuring a temperature control system which is inherently selfbalancing.

While I have shown and described two specific embodiments of my invention it will be understood that these are for purposes of illustration only and my invention is to be limited only by the scope of the appended claims.

I claim:

1. In combination, a source of light, the m tensity of which is to be maintained at a predetermined value, means for varying the intensity of said light, a control device for said intensity varying means, a photo-electric cell of the electron emissive type arranged to receive light from said source, a second photo-electric cell of the electron emissive type, an electric lamp illuminating said second photo-electric cell so as to maintain a substantially fixed ratio in the amounts of light supplied to said cells, thermionic means active upon a change in said ratio due to a change in the intensity of the source of light to vary the current supplied to said electric lamp to vary the illumination so as to restore said ratio in the amounts of light supplied to said cells, and further means responsive to a variation in the energy supplied to said illuminating means to causesaid intensity varying means to restore the intensity 'of the light to the predetermined value, said last named means comprising a pair of electro-magnetic coils, one of said coils being energized in accordance with the current flow through said lamp, the other of said coils being energized by a source of energy independent of the current fiow through said lamp, switching means controlled by said coils and adapted'to be in an open position when both of said coils are substantially equally energized, means responsive to movement of said switching mechanism from an open position due to an unbalance 01 said coils caused by a variation in current flow through the electric lamp to correspondingly vary the current supplied to the second of said coils, so as to restore the balance between said coils, and to simultaneously adjust the control device an amount proportional to said variation.

2. In a system for maintaining a temperature at a predetermined value, means forvarying the temperature, a source of electric current, an electrical circuit energized by said source of current and including means responsive to a change in temperature to vary the current fiow through said circuit, a switch movable in either direction from an intermediate open position to a closed position, a pair of electro-magnetic coils adapted upon equal energization thereof to hold said switch in said open position, and upon unbalance in the energization of said coils to move said switch to a closed position, one of said coils being energized by the fiow of current through said circuit, the other of said coils being energized by said source of current independently of said circuit, and means operable upon said switch being moved to a closed position by reason of an unbalance in the energization of said coils to vary the current flow through said second mentioned coil to restore the balance between said coils and to adjust said temperature varying means to cause the same to restore the temperature to substantially said predetermined value.

3. In a system for maintaining a temperature at a predetermined value, means for varying the temperature, a source of electric current, an electrical circuit energized by said source of current and including means responsive to a change in temperature to vary the current flow through said circuit, a switch movable in either direction from an intermediate open position to a closed position, a pair of electro-magnetic coils adapted upon equal energization thereof to hold said switch in said open position, and upon unbalance in the energization of said coils to move said switch to a closed position, one of said coils being energized by the flow of current through said circuit, the other of said coils being energized by said source of current independently of said circuit, a rheostat controlling the flow of current to said last mentioned coil, and a motor controlled by said switch and actuating said rheostat and said temperature varying means, said motor acting upon said switch being moved to a closed position by reason of the unbalance in the energization of said coils to vary the current flow through said mentioned coil to restore the balance between said coils and to adjust said temperature varying means to cause the same to restore the temperature to substantially said predetermined value.

4. In a system for maintaining a temperature at a predetermined value, means for varying the temperature, a source of electric current, an electrical circuit energized by said source of current and including means responsive to a change in temperature to vary the current flow through said circuit, a switch movable in either direction from an intermediate open position to a closed position, a plurality oi electro-magnetic coils, one or which is energized by the current flow through said circuit and at least two or which are energized by said source of current independently oi said circuit, the coils being located relative to said switch so that the first mentioned coil and one of the last mentioned coils act conjointly to urge the switch in one direction and the other of said coils urges the switch in the other direction, means for varying the current now through said two last mentioned coils to balance the action of all of the coils on said switch, and means operable upon said switch being moved to a closed position by reason oi-an unbalance in the energization of said coils to act on said current varying means to restore the balance of the coils and to act on said temperature varying means to restore the temperature to substantially said predetermined value.

5. In a system for maintaining a temperature at a predetermined value, means for varying the temperature, a source of electric current, an electricai circuit energized by said source of current and including means responsive to a change in temperature to vary the current flow through said circuit, a switch movable in either direction from an intermediate open position to a closed position, an electro-magnetic coil energized by the current flow through said circuit, a pair of electro-magnetic coils connected in series to said source of current independently or said circuit, a control potentiometer comprising a resistance and a sliding contact, said potentiometer being connected to said source or current in parallel with said pair of coils and the sliding contact of said potentiometer being connected to the junction of said coils, said potentiometer serving to control the distribution of the current flow through said coils, the first mentioned electro= magnetic coil and one of the second mentioned coils urging the switch in one direction and the other of said coils urging said switch in the other direction, and means controlled by said switch and operable upon said rheostat to maintain said coils balanced, and upon said temperature varying means to adjust said temperature.

6. In a system for controlling a condition, con:- dition varying means, a source of electric current, an electrical circuit energized by said source of current and including means responsive to a change in condition to vary the current flow through said circuit, a switch movable in either direction from an intermediate open position to a closed position, an electromagnetic coil energized by the current flow through said circuit, a pair of electro-magnetic coils connected in series to said source of current independently of said circuit, a control potentiometer comprising a resistance and a sliding contact, said potentiometer being connected to said source of current in parallel with said pair of coils and the sliding conand operable upon said rheostat to maintain said coils balanced, and upon said condition varying means to adjust said condition.

7. In a system for controlling a condition, condition varying means, a source of electric current, an electrical circuit energized by said source of current and including means responsive to a change in condition to vary the current flow through said circuit, a switch movable from an intermediate open position into either of two,

closed positions, an electro-magnetic coil energized by the current flow through said circuit, a pair of electromagnetic coils connected in series to said source oi current independently of said circiut, means for dividing the current flow through said pair of coils and ior varying said dvision of current flow, said first coil having an electro-magnetic efiect which is relatively small in comparison to that of either of said pair of cells, the first mentioned coil and one or said second mentioned coils urging the switch in one direction and the other of said coils urging the switch in the other direction, and means controlled by said switch and operable upon said rheostat to maintain said coils balanced and upon said condition varying means to adjust said condition.

8. In apparatus for controlling the temperature of a hot body, means for varying the temperature of said hot body, control means therefor, electrical motor means for varying the controlling position of said control means, a balanced relay for controlling said motor means and comprising two opposed relay windings, means including a photo electric device responsive'to the temperature of the hot body for controlling the energizetion of one of said windings and means controlled in accordance with the position of said control means for energizing the other winding, said relay being operative upon the same being unbalanced by a variation in the output of said photoelectric cell-due to a change in temperature of the hot body to cause said motor means to be energized in a direction and to an extent such that said controlling means is adjusted an amount proportional to such variation.

9. In condition changing apparatus, condition varying means, control means therefor, electrical motor means for adjusting the position of said control means, a balanced relay for controlling said motor means and comprising two oppose relay windings, means for energizing one of said relay windings comprising a grid controlled space discharge amplifier, an input grid circuit there= for comprising means responsive to said condition, and an output circuit electrically connected to said relay winding, and means controlled in accordance with the position of said control means for energizing the other oi said relay windings, said relay being operative upon the same being unbalanced by a variation in the output oi? said amplifier due to a change in said condition to cause said motor means to be energized in a direction and to an extent such that said control means is adjusted an amount proportional to said variation.

'10. In condition controlling apparatus, condition changing means employing a fluid medium, a valve for controlling the flow of said medium, reversible motor means for operating said valve, energizing means thereior, means connecting said motor to said energizing means including a balanced relay adapted when said relay is unbalanced in one direction to cause said motor to rotate in one direction to close said valve and when said relay is unbalanced in the other direction to cause said motor to rotate in the opposite direction, said relay comprising a pair of opposed relay windings, means for energizing one of said relay windings comprising a space discharge amplifier, an input circuit therefor comprising means responsive to said condition, and an output circuit electrically connected to said relay' winding, and means responsive to the position of said valve for energizing the other of said relay windings, said relay being operative upon the 'same being unbalanced by a variation in the output of said amplifier due to a change in said condition to cause said motor means to be energized in a direction and to an extent such that said valve is adjusted an amount proportional to said variation.

tensity of which is to be maintained at a predetermined value, means ior varying the intensity of said light, a photo-electric cell of the electron emissive type arranged to receive light from said source, a second photo-electric cell or the electron emissive type, means for illuminating said second photo-electric cell to maintain a substantially fixed ratio in the amounts of light supplied to said cells, means responsive to a change in the intensity of said source of light with the resultant change in said ratio to vary the energy supplied to said illuminating means to restore said ratio in the amounts oi light applied to said cells, further means responsive to a variation in the energy supplied to said illuminating means to cause said intensity varying means to vary the 11. In combination, a source of light the inintensity of said source of light to restore the value thereof to the predetermined value, said last named means comprising a control device for said intensity varying means, motor means for operating said control device, control means for said motor means operative as a result of a variation in the energy supplied to said illuminating means to cause said motor means to be energized in a direction and to an extent such that said control device is adjusted an amount proportional to said variation in energy supplied to said illuminating means.

12. In combination with a heating device, a photo-electric unit of the electron emissive type arranged to be subjected to the light from a body heated by said device, a second electrically energized source of light, a second photo-electric unit of the electron emissive type arranged to receive light from said second source of light, a grid controlled space discharge device, energizing means, and connections between said energizing means, said photo-electric units, and said space discharge device operative to vary the grid bias or said device upon a change in the relative amounts or light received by said units, means operative to maintain a definite relationship between the energization of said second source 01 light and the output of said space discharge device, and means operative to control said heating device in accordance with the energization of said second source of light, said last named means comprising a control device for said heating device, motor means for operating said control device, control means for said motor means responsive to the energization of said second light source and operative upon a variation in such energization to cause said motor means to be energized in a direction and to an extent such that said control device is adjusted an amount proportional to said variation.

PAUL I". SHIVERS. 

